I have a Wilcox White organ combination pushup with 10 stops.  I have
one with three controls at front, one is missing.  Those I have are

  Tempo -- speeds marked largo, presto, etc.
  Loudness -- from pp to ff.

Does anyone know what the other control might be?  The forward/reverse
control is on one of the organ stops, so it is not that.

I am also looking for a pair of wheels/castors on a frame which are
missing.  I would happily buy any book about these instruments but
suspect I may have to write one myself!

Thanks, Kevin McElhone, Northants., UK